1. Field of the Invention
The present invention relates to a moving image encoding apparatus, and more particularly to a moving image encoding apparatus for use with a visual telephone system or a video conference system.
2. Description of the Related Art
Conventionally, a moving picture encoding system which employs motion compensation and orthogonal transform in combination is used frequently for compression coding of a moving image signal. Meanwhile, an input image signal inputted to an encoding apparatus includes high frequency components of noise, and in order to prevent deterioration of the coding efficiency by such high frequency components, a pre-processing filter is used to eliminate noise.
As such pre-processing filters, a recursive interframe filter for suppressing temporal noise components and an intraframe filter for suppressing spatial noise components are known. For a recursive interframe filter, a first-order recursive filter for pixels which includes a frame memory for one frame is widely used, and for an intraframe filter, a two-dimensional filter of 3xc3x973 elements is widely used. Moving image encoding apparatus which employ such pre-processing filters as described above are disclosed in Japanese Patent Laid-Open No. 154588/1990, Japanese Patent Laid-Open No. 84024/1997, and so forth.
FIG. 9 shows an example of a moving image encoding apparatus which employs a pre-processing filter mentioned above. Conventional moving image compression coding is described below with reference to FIG. 9. The moving image encoding apparatus shown processes an input image 101 in units of a pixel block (for example, 16xc3x9716 pixels or 8xc3x978 pixels).
In particular, a pre-processing filter circuit 21 filters a pixel block of the input image 101 based on a result of discrimination of a moving/still discrimination circuit 22 and outputs a result of the filtering to a subtractor 1. The subtractor 1 subtracts a predicted image 102 outputted from a motion compensation circuit 10 from the pixel block filtered by the pre-processing filter circuit 21.
A result of the subtraction of the subtractor 1 is inputted through a DCT circuit 23 and a quantization circuit 24 to an encoding circuit 4, by which it is encoded, and an output of the encoding circuit 4 is stored into a buffer 5.
The output of the quantization circuit 24 is inputted not only to the encoding circuit 4 but also to another adder 8 through a dequantization circuit 6 and an inverse DCT circuit 7. The adder 8 adds the predicted image 102 to the output of the quantization circuit 24, that is, an output of the inverse DCT circuit 7, and an output of the adder 8 is stored into a frame memory 9.
A quantization step size 103 which is used by the quantization circuit 24 is determined by an encoding control circuit 25. The quantization step size 103 used by the quantization circuit 24 is inputted also to the encoding circuit 4 and the dequantization circuit 6 simultaneously.
The encoding circuit 4 encodes the quantization step size 103 into coded data while the dequantization circuit 6 performs dequantization with the quantization step size 103.
Referring to FIG. 10, the quantization circuit 24 includes a multiplier 41, a rounding circuit 42, and a multiplication coefficient table 43. A quantization step size 103 inputted to the quantization circuit 24 is varied in units of a pixel block, but is constant during processing of the same pixel block.
The multiplication coefficient table 43 includes a correspondence between the quantization step size 103 and a multiplication coefficient 401 which is used by the multiplier 41. For example, where the dequantization circuit 6 should output a result of multiplication of an input thereto by the quantization step size 103, the multiplication coefficient table 43 in the quantization circuit 24 is set so that the input is multiplied by an inverse number to the quantization step size 103.
At the top of processing of a pixel block, a multiplication coefficient 401 corresponding to the input of the quantization step size 103 is set to the multiplier 41. After the multiplication coefficient 401 is set, an input to the quantization circuit 24 is multiplied by the multiplication coefficient 401 by the multiplier 41. Then, an output of the multiplier 41 is rounded by the rounding circuit 42, and an output of the rounding circuit 42 is outputted as an output of the quantization circuit 24.
In the conventional moving image encoding apparatus described above, noise elimination from an input image signal in moving image encoding processing, particularly in encoding processing at a low bit rate, is an important factor to assure a high encoding efficiency. However, the conventional moving image encoding apparatus is disadvantageous in that a large amount of calculation is required for implementation of a pre-processing filter and this requires a large apparatus scale.
It is an object of the present invention to provide a moving image encoding apparatus which has a similar function to noise elimination by a pre-processing filter without requiring a large amount of calculation and a large apparatus scale.
In order to attain the object described above, according to the present invention, there is provided a moving image encoding apparatus, comprising prediction means for predicting a pixel block in a current frame of an input moving image signal from a preceding frame of the input moving image signal using motion compensation to produce a predictive error frame, orthogonal transform means for orthogonally transforming the predictive error frame, quantization means for quantizing the orthogonally transformed frame, encoding means for encoding the quantized frame, and a transmission buffer for storing the frame encoded by the encoding means, the quantization means quantizing the orthogonally transformed frame from the orthogonal transform means with a quantization step size having a different characteristic from that of a quantization step size which is used in dequantization included in the prediction processing of the prediction means and is used for the encoding processing by the encoding means.
Preferably, the moving image encoding apparatus further comprises moving/still discrimination means for detecting a moving area and a still area of the input image signal, and control means for controlling the quantization characteristic of the quantization means based on an output of the moving/still discrimination means and a remaining capacity of the transmission buffer.
In the moving image encoding apparatus, the quantization means quantizes the orthogonally transformed frame from the orthogonal transform means with a quantization step size having a different characteristic from that of a quantization step size which is used in dequantization included in the prediction processing, particularly, in motion compensation, of the prediction means and is used for the encoding processing by the encoding means. Consequently, the moving image encoding apparatus is advantageous in that it can be provided with a similar function to noise elimination by a pre-processing filter without requiring a large amount of calculation and a large apparatus scale.
Preferably, the quantization means quantizes the orthogonally transformed frame from the orthogonal transform means with a step size with which the orthogonally transformed frame is attenuated different from that which is used for the encoding operation by the encoding means. With the moving image encoding apparatus, a temporal variation of the input image becomes less likely to be encoded, and a temporal noise component elimination effect similar to that achieved where a recursive interframe filter is used can be achieved without using a recursive interframe filter.
Preferably, the quantization means quantizes the orthogonally transformed frame from the orthogonal transform means while attenuating high frequency components of the orthogonally transformed frame in higher degrees than low frequency components of the orthogonally transformed frame. With the moving image encoding apparatus, high frequency components of a spatial variation of the input signal become less likely to be encoded, and a spatial noise elimination effect similar to that achieved where an intraframe filter is used can be achieved without using an intraframe filter.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols.